The present invention relates to industrial processing plants. More particularly, the present invention relates to transmitters used to measure process variables in such plants.
Typically, a transmitter senses a characteristic, such as pressure, of a process fluid and then transmits an output proportional to the characteristic to a remote location over a plant communication bus. The plant communication bus typically uses a 4-20 mA analog current loop or a digitally encoded serial protocol such as HART or Fieldbus. Data, such as commands for alarms or setting span, zero, upper range level (URL), lower range level (LRL) setting, and the like can also be communicated to and from the transmitter via the plant communication bus using the digitally encoded protocol.
The transmitter has a housing with sealed flameproof compartments for use in process plants where flammable atmospheres may be present. The housing has an electronics compartment with a removable cover that seals and protects an embedded microprocessor, a sensor, and momentary xe2x80x9cON-OFFxe2x80x9d type switches for span and zero (or URL and LRL) settings. The housing also has a field wiring compartment with a removable cover that protects and seals field wiring connections with the plant communication bus. Electrical connections between the electronics and field wiring compartments are made using bulky sealed electrical feedthroughs that ensure flameproofing between the compartments.
Also, in some fluid processing plants, there is existing electrical equipment, such as limit switches or controllers, that are not provided with a digitally encoded output such as HART or Fieldbus, but instead have only simple xe2x80x9cON-OFFxe2x80x9d or switch type inputs or outputs. There is a desire to communicate these xe2x80x9cON-OFFxe2x80x9d type inputs or outputs, if used, to the microprocessor in the sealed sensor module.
Feedthrough contacts needed to accommodate all of the xe2x80x9cON-OFFxe2x80x9d inputs and xe2x80x9cON-OFFxe2x80x9d outputs are bulky and would increase the transmitter housings to such a large size that the transmitter would become unreasonably heavy and expensive for many desired process applications.
A sensor module for use in a transmitter includes a logic circuit having a selectable first mode of operation in which the logic circuit generates ON-OFF logic levels on two logic level contacts on the sensor module, and having a selectable second mode of operation in which the logic circuit detects ON-OFF logic levels on the same two shared logic level contacts.
The sensor module has an outer module housing with an inlet adapted for fluid communication with a fluid, and an electrical feedthrough with contacts.
The sensor module also has a transducer circuit coupled to the logic circuit. The transducer circuit includes a regulator coupled to energization contacts, a sensor sensing a fluid characteristic, and a transceiver adapted to couple a serial representation of a sensed fluid characteristic to a serial output contact.
The logic circuit can be conveniently set to select either generating ON-OFF logic level outputs or detecting ON-OFF logic level inputs, depending on the needs of the application. The ON-OFF logic level inputs share contacts with the ON-OFF logic level outputs and the use of additional dedicated feedthrough contacts for the logic level outputs is avoided.